Resistive Random Access Memories (RRAM) are today the subject of great interest, particularly on account of their low electrical consumption and their high operating speed.
A resistive type memory cell has at least two states: a “High Resistance State” (HRS), also called “OFF” state, and a “Low Resistance State” (LRS) or “ON” state. It may thus be used to store binary information.
Three types of resistive memories may be distinguished: memories based on thermochemical mechanism, memories based on valence change, and memories based on electrochemical metallisation.
The field of the present invention more particularly relates to this latter category based on ion conduction materials (CBRAM or “Conductive Bridging RAM” memories). The operation resides in the reversible formation and rupture of a conductive filament in a solid electrolyte, through dissolution of a soluble electrode. These memories are promising due to their low programming voltages (of the order of a Volt), their short programming time (<1 μs), their low consumption and their low integration cost. Furthermore, these memories can be integrated into the metallisation levels of the logic of a circuit (“above IC”), which makes it possible to increase the integration density of the circuit. From the architectural viewpoint, they only require a selection device, a transistor or a diode for example.
The operation of CBRAM memories is based on the formation, within a solid electrolyte, of one or more metal filaments (also called “dendrites”) between two electrodes, when said electrodes are taken to suitable potentials. The formation of the filament makes it possible to obtain a given electrical conduction between the two electrodes. By modifying the potentials applied to the electrodes, it is possible to modify the distribution of the filament, and thus to modify the electrical conduction between the two electrodes. For example, by reversing the potential between the electrodes, it is possible to make disappear or reduce the conductive filament, so as to eliminate or reduce considerably the electrical conduction due to the presence of the filament.
FIGS. 1a, 1b and 1c schematically illustrate the operation of a CBRAM type memory device 1. The memory device 1 is formed by a stack of Metal/Ion conductor/Metal type. It comprises a solid electrolyte 2, for example based on doped chalcogenide (e.g. GeS) or oxide (e.g. Al2O3). The electrolyte 2 is arranged between a lower electrode 3, for example made of Pt, forming an inert cathode, and an upper electrode 4 comprising a portion of ionisable metal, for example copper, and forming a soluble anode. A portion of ionisable metal is a portion of metal able to form metal ions, for example Cu2+ ions in the case where the ionisable metal is copper, when it is subjected to a suitable electrical potential. The memory device 1 represented in FIGS. 1a, 1b and 1c typically forms a memory point, that is to say a unit memory cell, of a memory comprising a multitude of these memory devices.
FIG. 1a schematically illustrates the memory device 1 in the virgin state, before the first use of said memory device 1, that is to say before the first application of a potential difference between the soluble electrode 4 and the inert electrode 3 for the passage of the memory device 1 to the “ON” state. FIG. 1b schematically illustrates the memory device 1 in the “ON” state. FIG. 1c schematically illustrates the memory device 1 in the “OFF” state.
The first use of the memory device 1 makes it possible to pass from the virgin state to the “ON” state, thanks to the carrying out of a step called “forming”. The step of passing from the “ON” state to the “OFF” state is called “RESET”, whereas the step of passing from the “OFF” state to the “ON” state is called “SET”.
When a potential difference is applied between the soluble electrode 4 and the inert electrode 3, the electrical potential applied to the soluble electrode 4 being greater than the electrical potential applied to the inert electrode 3, an oxidation-reduction reaction takes place at the soluble electrode 4, creating mobile ions. In the case of a soluble copper electrode 4, the following reaction takes place:|Cu→Cu2++2 e−.|[E1]
The mobile ions then move in the electrolyte 2 under the effect of the potential difference applied between the electrodes. The speed of movement depends on the mobility of the ion in the electrolyte in question, which guides the choice of the soluble electrode/electrolyte pairing (examples: Ag/GeS; Cu/Al2O3, etc.). The speeds of movement of the ions are of the order of nm/ns. On arrival at the inert electrode 3, the mobile ions are reduced due to the presence of electrons supplied by the electrode 3, leading to the growth of a conductive filament 5 according to the following reaction:Cu2++2 e−→Cu
The conductive filament 5 grows preferentially in the direction of the soluble electrode 4. The memory device 1 passes to the “ON” state when the filament 6 enables contact between the electrodes 3 and 4, making the stack conductive. FIG. 1b schematically illustrates the memory device 1 in the “ON” state.
A potential difference applied between the soluble electrode 4 and the inert electrode 3, the electrical potential applied to the soluble electrode 4 being less than the electrical potential applied to the inert electrode 3, instead leads to the dissolution of the conductive filament 5. To justify this dissolution, thermal mechanisms (heating) and oxidation-reduction mechanisms are generally invoked. The memory device 1 then passes to the “OFF” state. Often, the electrolyte 2 contains in the “OFF” state a residual filament 6, which is in contact with the cathode 3 but which is not in contact with the anode 4. The residual filament 6 stems from an incomplete dissolution of the conductive filament 5. A filament is called residual when it does not establish a sufficient electrical conduction between the electrodes to obtain the “ON” state. FIG. 1c schematically illustrates the memory device 1 in the “OFF” state.
During the first use of the memory device 1, that is to say during the first application of a potential difference between the soluble electrode 4 and the inert electrode 3, the conductive filament 6 is generated for the first time: it is the forming step mentioned previously, which makes is possible to pass from the “virgin” state to the “ON” state. The potential difference required to carry out this first forming step is typically greater than the potential difference required thereafter during SET steps. Furthermore, the potential difference necessary to carry out the forming step can vary from one memory device to the next. In a memory array comprising a plurality of memory devices, there thus exists typically a distribution, a dispersion of the potential difference to apply to each memory device, for the first formation of the conductive filament within each memory device. This dispersion is explained in particular by the fact that the potential difference required to carry out the first forming step is a function of the initial resistance value, noted R0, of the memory device 1 in the virgin state. In a memory array comprising a plurality of memory devices, the distribution of the initial resistance value R0 typically attains several decades. During the forming step, it is nevertheless very important to apply to each memory device a potential difference which is adjusted to it. In fact, the application of a potential difference less than the nominal potential difference does not enable the first formation of the conductive filament. Instead, the application of a potential difference greater than the nominal potential difference leads to a degradation of the electrical performances of the memory device, particularly in terms of reliability, retention and endurance.
In order to avoid the application of a too great potential difference to a memory device during the forming step, a solution consists in carrying out successively several forming cycles with increasing potential differences. The state of the memory device is read after each cycle in order to determine its state, that is to say in order to know whether the conductive filament is formed or not. If the conductive filament is not formed, the following cycle is carried out. If the conductive filament is formed, the process is stopped for the memory device considered. Such a solution nevertheless implies a long forming time, particularly in the case of a memory array comprising a plurality of memory devices, the potential difference to apply to each memory device having to be adjusted for each of said memory devices.